Electrooptical feedback system



Jan. 21, 1947. A. BADMAIEFF ELECTRO-OPTICAL FEEDBACK SYSTEM Filed Nov.5o, 1942 2 sheets-sheet 1 ATTORNEY.

Jan. 2l, 1947, v A, BADMAIEFF 2,414,686

ELECTRO-OPTICAL FEEDBACK SYSTEM Filed Nov. so, 1942 2 Sheets-sheet 2 @MS P 4141( 4 wm.) I. I l XMQQQW u LQQSR A MAMMA vvvvvv y ALEX/5 5AoMA/EF5INVENTOR.

BY ATTORNEY.

atented Jan. 21, 1947 ELECTROOPTICAL FEEDBACK SYSTEM Alexis Badmaiel,Indianapolis, Ind., assignor to Radio Corporation of America, acorporation of Delaware Application November 30, 1942, Serial No.467,442

(Cl. Z50-41.5)

13 Claims.

This invention relates to photographic sound recording and reproducingsystems and particularly to a system for stabilizing the light sourcesused in such systems. Y

It is well known that in photographic sound recording systems, lightsources such as lamps are employed to produce light beams which may beeither directly modulated by varying the energy supplied to the lamps inaccordance With sound waves to be recorded or which may be modulated byvibrating the beam in accordance with the sound waves. Light sources ofconstant intensity are used for recording both variable area andvariable density sound tracks, the light beam being vibrated by agalvanometer mirror. Light sources which vary in intensity are, however,usually employed for recording variable density records. For reproducingeither type of record, a light source of constant intensity is used toproduce the scanning beam. In all cases, however, the light output ofthe source should remain constant and uniform during the recording orreproducing period to prevent undesired variations in the sound levelsand the introduction of distortion in the recorded or reproduced sound.

The present invention is directed to a feedback circuit which willcompensate for variations in the light intensity due to Variations inthe supply energy, blackening of the lamp envelope, deterioration in thelight producing elements of the lamp or anything tending to change thelight intensity from a predetermined value. Systems of this general typeare known in the art, one being disclosed in Balsley Patent No.2,242,638 of May 20, 1941. In this patent the electrical energy for thelamp is connected in series with a Vacuum tube, the impedance of whichis varied by the variations in light intensity, thus providing directcurrent feedback. In this arrangement, `the amount of energy which canbe supplied to the light source is limited by the size of the vacuumtube in series with it. Another light source stabilizing system isdisclosed and claimed in Collins Patent No. 2,274,530 of February 24,1942, in which an opening in a shutter is controlled by the lightintensity.

'I'he present invention is directed to a light stabilizing system whichutilizes two sources of energy for the light source, one being a primarysource and the other a Asource varying in accordance with lamp intensityto compensate for variations in the light output of the lamp. Sincethese energy sources are connected in series and the primary sourcesupplies the major amount of energy, the control circuit may be used forlamps having a. Wide range of light outputs.

Another feature of the invention is the use of an oscillator as thesecondary source of energy, the rectified output thereof beingcontrolled by the fluctuations in light output of the lamp source. Thisprovides a sensitive and quick acting control of the energy supplied tothe lamp while amplification of the detected variations is simple andefficient.

The principal object of the invention, therefore, is to improve opticalfeedback stabilizing systems for light sources.

Another object of the invention is to provide an optical feedbackstabilizing` system which is applicable to light sources of differentcapacities.

A further object of the invention is to provide an optical feedbacksystem for stabilizing a light `source which is capable of responding torapid changes in light output as well as to slow variations therein.

A further object of the invention is to provide an optical feedbacksystem for stabilizing a light source which utilizes a high frequencyoscillator whose output is varied in accordance with the variations inthe light intensity of said source.

Although the novel features which are believed to be characteristic ofthis invention are pointed out with particularity in the claims appendedherewith, the manner of its organization and the mode of its operationwill be better understood by referring to the following description,read in conjunction with the accompanying drawings, in which:

Figure 1 is a block diagram of a light stabilizing system embodying theinvention;

Figure 2 is a schematic circuit diagram of the system of Fig. 1; and lFigs. 3 and 4 are schematic diagrams of modification details of thecircuit of Fig. 2.

Referring now t0 Fig. 1, a light source 5 of the two electrode gaseoustype is adapted to have its light output maintained constant. A portionof the light from the source 5 is detected by a photoelectric cell 6,the output of which is impressed on a variable gain amplifier l. Theinput to the variable gain amplier 1 is connected to an oscillator 9while the output is connected to a power amplier l0. The output currentfrom the amplifier l0 is rectified by a rectier Il and then impressedupon the light source 5 in series with a primary direct current powersource connected across terminals I3, the output of rectifier Il beingfiltered by a condenser I4. In general, therefore, the light source 5 issupplied with energy from both a fixed primary direct current source anda controllable secondary source, the latter being the oscillator 9, theoutput of which is rectified by the rectifier Il. Should the lightquanta of lamp decrease, the gain of the variable gain amplifier 1 isincreased by the decreased output from cell 5, and the rectier IIincreases its energy supply to the lamp 5. Should the light quanta fromlamp 5 increase, the rectifier output is proportionally decreased toreturn the lamp to its predetermined adjustment.

In Fig. 2 the details of the system of Fig. l are shown, the lightsource 5 illuminating photocell l as in Fig. 1. In this circuit thephotoelectric cell G is connected over conductors 2li to the input of aVariable gain amplifier tube I5 across an input resistance I6. Biasingresistance Il shunted by a by-pass condenser I9 are shown in the cathodereturn circuit while a resistance I8 is provided with a variable tap forcontrolling ,the output of cell 6. The resistancel is shunted by avoltage regulator tube 2 I.

An oscillator 23 with its tank circuit 24 and feedback inductance 25 iscoupled through condenserZ'I to the control grid of tube I5. Theoscillator is provided with a biasing resistor 28 shunted by a by-passcondenser 29. The output of the tube i5 is impressed, through condenser3l, upon a power amplifier tube 30, the output of amplifier 30 beingconnected to the primary of a transformer 32. The low impedancesecondary of transformer 32 is connected to any suitable type ofrectifier 33 such as RCA type 5Z4 or a full-wave Rect-,ox dry rectifier,the rectifier being loaded by a resistance 35 which is in series withconductors 35 of the main direct current supply source connected acrossterminals I3. The ampliers l5 and 30 and oscillator 23 are suppliedywith plate po-tentials from any suitable direct current supply sourceconnectable to terminals 3c over potentiometer resi-stances 39 and 1li,as is standard practice, resistance dl being tapped to vary the anodepotential on cell 6.

vFrom the above it is to be noted that the oscillator 23 no-rmallysupplies a constant output to the amplier I5 which produces a certainvoltage across resistance 35 in series with the main direct currentsupply source connected at terminals i3. To prevent any oscillatorripple reaching the voltage supply to cause fluctuations in the lightsource, the oscillator frequency is preferably adjusted to be in theneighborhood of 20,009 cycles, which frequency is doubled by thefull-wave rectifier and filtered by condenser i4. Any frequency between15,000 and 100,000 cycles will be satisfactory, however, the highfrequency limit being that fixed by the distributed capacity in therectifier if it is a dry disc type. The voltage created acrossresistance 35 is additive to that connected across terminals I3 so thatthe sum of voltages of each source is applied to the lamp 5.

N-o-w, should the light falling on cell 5 from lamp 5 decrease inquanta, the gain of amplifier I5 is increased by the decreased output ofcell 6 and the increased output of amplifier I5 is impressed upon poweramplifier S which increases the rectified output of the rectifier 33.The increased output of the rectifier increases the Voltage acrossresistance 36, thus supplying more energy to the lamp 5 to compensatefor the decrease in light quanta. Likewise, any increase in lightoutputdue to an increase in volttage across iterminals I3 or for otherreasons, will decrease the gain of amplifier I5 which willcorrespondingly decrease the voltage across resistance 36 to bring thelamp 5 back to its normal output.

In the above system the voltage variations obtained across resistor 36will follow either slow or fast changes in the light variations fallingon the photoelectric cell 5 and the system will, therefore, resp-ond toboth alternating current and direct current components, to provide aparticularly sensitive stabilizing system. By utilizing the light outputof the lamp for controlling its output, the system will functionregardless of Iwhether or not the cause of the variations in light aredue to energy variations in the primary source, blackening of the lampenvelope or other reasons. Furthermore, since the major portion of theenergy for the lamp is from an independent source, lamps having a widerange of capacities may be controlled.

Referring now to Fig. 3, the input circuit of variable gain amplifiertube I5 is shown as including resistances 50 and 4I in the cathodecircuit. With this modification of Fig-2, the lamp 5 may be bothmodulated with an audio signal across resistance 40 and a noisereduction signal applied across resistance il, ,for sound recordingpurposes. Stabilization and a reduction of harmonic distortion due tonon-linearity of the lamp will be o-btained during recording by theoptical negative feedback action as described above.

In Fig. 4 a power tube 44 is shown connected between the lamp 5 and therectifier load resistance 36 and in series with the primary source ofpower. The power tube 43 functions asa variable resistor inseries withthe lamp to control it. It is to be understood that with a sulficientlylarge secondary power source the primary source across terminals I3 maybe eliminated. Furthermore, with a heated filament type of lamp 5 therectifier may be eliminated since the lilament will not be sensitive topolarity reversals and if the frequency is high enough, the filament 7will not follow theoscillator -frequency.

I claim as my invention:

1. The method of controlling the light quanta from a light sourcecomprising energizing said light source from a circuit including adirect current energy supply, translating a portion of the light fromsaid light source into corresponding electrical voltages, generatingfrom a separate energy supply a, substantially constant amplitude highfrequency current for further energizingsaid light source circuit,amplifying Said highV frequency current and controlling by saidtranslated voltages the amplification and impression of said highfrequency current on said vlight source c ircuit to maintain saidlightquanta at a substantially constant value. 'Y

2. The method of maintainingA the light output of a light sourcesubstantially constant comprising energizing said light source from acircuit including a direct current source of energy, gen-..

erating from a separate energy source a high frequency current ofsubstantially constant amplitude, amplifying said high frequencycurrent, rectifying said high frequency current, impressing the productsof rectification on said light source circuit, translating a portion ofthe light from said source intoV electrical voltages, 'and controllingby said translated voltages the amplification of .said high frequencycurrent and the ampiitude. of l the. prduct's @rectification tomamtainsaid light output at a substantially constant value.

3. The method of maintaining the light output of a light sourcesubstantially constant comprising energizing said light source from acircuit including a direct current source of energy, generating from aseparate energy source a high frequency current of substantiallyconstant amplitude, amplifying said high frequency current, rectifyingsaid high frequency current, further energizing said light source withsaid rectified current, detecting a portion of the light output of saidlight source, and controlling the amplification of said high frequencycurrent with said detected light variations in a direction and in anamount to maintain the light output of said light source substantiallyconstant.

e. The method in accordance with claim 3, in which the frequency of saidhigh frequency current is above 15,000 cycles and below 100,000 cycles.

5. The method in accordance with claim 3, in which the light output ofsaid light source is modulated by audio signals,

6. .A light source stabilizing system comprising a light source, adirect current source of energy for said light source, a rectifierhaving its output connected in series with said source of energy andsaid light source, a separate source of energy comprising an oscillator,an amplifier connected between the output of said oscillator and theinput of said rectifier, and means connected to the input of saidamplifier for translating a portion of the light from said light sourceinto corresponding electrical voltages for controlling the gain of saidamplifier and the amplification of the output of said oscillator asimpressed on said rectifier', variations in said translated lightproducing in- Verse Variations in the energy supplied to said lightsource from said oscillator.

7. A light source stabilizing system in accordance with claim 6, inwhich said high frequency oscillator generates currents of approximately20,000 cycles.

8. A stabilizing system for a light source comprising a light source, adirect current source of energy for said light source, a separate sourceof alternating current energy, means connecting said alternating currentsource of energy and said light source for rectifying said alternatingcurrent source of energy before impression on said light source, meansfor detecting light from said source, an amplifier connectedintermediate said source of alternating current energy and saidrectifying means and adapted to amplify said alternating current beforeimpression on said rectifying means, and means interconnecting saidlight detecting means and said amplifier for controlling the gain ofsaid amplifier by the output voltage of said detecting means.

9. A stabilizing system in accordance with claim 8, in which saidalternating current source is a relatively high frequency oscillatoradapted to generate currents of frequencies between 15,000 and 100,000cycles.

10. A stabilizing `system in accordance with claim 8, in which means areprovided for impressing an audio signal on said amplier for modulatingsaid light source.

11. An optical feedbacksystem for stabilizing a light source comprisinga light source, means for translating a portion of the light from saidsource into electrical voltages, a direct current energy sourceconnected to said light source, separate means for independentlygenerating an alternating current, means for rectifying said alternatingcurrent, means for further energizing said -light source with saidrectified current, said rectifying means interconnecting said lastmentioned means and said generating means, and means connectedintermediate said translating means and said generating means forvarying the amplification of said alternating current and therebycontrolling the amountA of said rectified current utilized forenergizing said light source.

12. An optical feedback system in accordance with claim 11, in Whichsaid generating means includes a relatively high frequency oscillatorgenerating currents of approximately 20,000 kilocycles.

13. An optical feedback system in accordance with claim 11, in whichsaid last mentioned means includes means for impressing a signal thereonfor modulating said light source.

ALEXIS BADMAIEFF.

